One area of
interest to a lot of Canon EOS camera users involves attaching old manual
lenses and telescopes to their autofocus cameras. Canon dont say anything
about this in their manuals - they just include a warning
about how using non-Canon lenses might somehow ruin your camera. While this
may theoretically be true, in real life its not a problem. And opting for non-Canon
lenses opens up all kinds of interesting choices to you.

In fact, it’s really common for people using digital EOS cameras for video production to use Nikon glass. And a few years ago the animated stop-motion feature film Tim Burton’s Corpse Bride was shot using precisely this combination: a bunch of Canon EOS 1D mark II cameras equipped with various Nikon lenses.

Now a number of
third-party manufacturers build autofocus lenses which are designed to be
fully compatible with Canon EOS cameras and their EF (electrofocus)
lens mount system - Tamron, Tokina and Sigma being the big three. On the whole
these third-party lenses are quite compatible with EOS cameras. A number
of older Sigma lenses cause newer EOS cameras to lock up temporarily, but
Ive personally never heard of any third-party EF-mount lens permanently damaging
a camera, though I suppose anything is possible.

But what about
lenses meant for other camera mounts? Well, many such lenses can be adapted
to Canon EOS cameras through the use of special adapter rings. Not all, of
course. For example, a lens designed to work with a non-Canon EOS autofocus
camera (eg: an autofocus Nikon, Pentax or Minolta camera) isnt going
to work with an EOS camera very well since the electronics are totally incompatible.
However, older fully-mechanical manual lenses can often be adapted successfully
to EOS cameras. So can many telescopes and spotting scopes. Heck - so can
magnifying
glasses attached to EOS bodies with cardboard tubes and a lot of tape! And you can even buy all-manual lenses with EF lens mounts built in, such as the Lensbaby selective-focus lenses, and the Loreo Lens in a Cap.

Of course, certain
issues arise in using these manual lenses, and thats what this document
is about.

Note that this
document does not deal with the four Canon-built manual-focus lenses currently
in the EOS-compatible lens lineup - the three TS-E tilt-shift lenses and the MP-E 65mm
5x macro lens. Although these exotic and highly specialized lenses lack autofocus
motors (and hence lack the official EF appellation since they are not "electrofocus") they nonetheless have EF lens mounts, EOS-compatible electronics and
electronically-controlled aperture diaphragms.

Adapter
rings.

First of all
you have to buy the appropriate physical lens adapter required to get your
lens attached to your camera. For this you should consult your favourite camera
shop - I cant help you here. But a number of manufacturers build simple
metal ring adapters that let you attach various types of lenses. Robert Monaghans
page, listed at the end of this document, mentions a few retailers and manufacturers
of such adapters.

If youve
got, for example, a lens designed for an old Pentax threaded (screw mount)
camera like the Pentax Spotmatic youll need an adapter for M42 lenses.
Same with the new line of Carl Zeiss ZS lenses, which also use M42.
If you have a telescope youll probably need a T-mount adapter, assuming
your telescope has the ability to be hooked up to a camera at all. Or if you
have a Nikon lens youll need an adapter that lets you fit Nikon F lenses
to your EOS body. Likewise Leica R, Leica Visoflex, Olympus OM, Contax/Yashica RTS, Rollei, and Contax manual focus, or
even big Hasselblad, Mamiya 645, Pentacon 66, Pentax 645 and Pentax 6x7 medium
format lenses can all be adapted. Microscopes apparently often require C-mount adapters. Tamron
built a series of Adaptall II lenses and you can still occasionally find used
Adaptall II->EOS adapters around. I have links to some sources for these
adapters at the end of the page.

Most adapters
are simply metal rings with an EF bayonet on one side; usually worth around
$20-50 US. And since the EF mounting ring is larger in diameter than most 35mm
SLR lens mounts quite a few lenses can be adapted to the EOS system.

Adapter ring compatibility problems and infinity focus

The
most common problem is that of lens registration - the distance from the lens mount to
the film plane. It may not be possible to build a lens adapter that gets this
distance right for certain lens mount systems.

For example, the back focus distance (distance from the rearmost lens element to the film plane) on EOS cameras is 44mm, but on Leica rangefinder cameras its 27.8mm. Matching the back focus distance would require a lens mounted deep inside the camera body; not usually an option. In cases such as this you may either have to buy an adapter ring that
wont let you achieve infinity focus (fine for macro photography; probably
not fine for anything else) or youll have to get an adapter ring which
contains one or more glass lens elements and which will inevitably degrade the image quality somewhat. These compensating elements let
you achieve focus to infinity, though at the expense of image quality. Contax G, Konica F, Leica M, Minolta MD and Miranda lenses are other lens mounts with back focus distances less than that of Canon EOS.

There
is also the case of lenses which have lens register distances very close to
that of a true Canon EF lens. Contax/Yashica lenses (45.5mm) and Canon EF lenses (44mm), for example, have only a 1.5mm difference in lens registration, and 1.5 is awfully thin for a metal ring adapter. Such adapters have to be machined to incredibly close tolerances in order for infinity focus to work, which means they can be quite expensive. Cheaper ones often have to be sanded down to maintain infinity focus - but even that is something of a dodgy proposition as you have to sand the surface down accurately from one side to the other, down to a fraction of a millimetre.

Sadly it also appears
that it isnt possible to make adapter rings that let you mount Pentax
K (bayonet mount) lenses on EOS cameras with EF lens mounts. The Pentax K diaphragm coupling lever physically interferes with
such a possibility. So youd have to modify the lens (or the camera)
to make such a conversion possible. Note that this does not apply to cameras compatible with the EF-S mount. The design of the EF-S mount, with its smaller mirror box thanks to the 1.6x crop factor, coincidentally permits the use of such lens adapters.

Finally, and
this isnt strictly to do with manual focus lenses per se, but you cant use
Minolta Vectis or Four Thirds (Olympus, Kodak, Fuji, Sanyo, Sigma, Panasonic and Leica) lenses on EOS cameras.
These lenses, in addition to being autofocus lenses, cast very small image
circles; much too small to cover the imaging area of EOS cameras. In theory I suppose the Vectis lens design might cover an EF-S sensor, but it would probably be pointlessly expensive to make such an adapter.

Problems
with Canon FD manual lenses.

Canon FD lenses
are manual-focus only lenses which Canon sold in the years before switching
over to the autofocus EF system. Many, particularly those made in the late
70s and early 80s, offer excellent optical quality, have smoothly-operating
metal barrels and are available quite inexpensively on the used market. So
the obvious thought comes to mind - can such lenses be attached to EOS cameras?

Unfortunately
the lens mounts used by the two systems are completely incompatible. FD lens
mounts are smaller in diameter, have a different lens register from EF lenses,
rely on mechanical levers to control lens aperture, are of a breech-lock design
(rotating pressure ring) and never contain autofocus motors.* EF lens mounts
are larger in diameter, are of a bayonet mount design (put lens into camera
and rotate partway to lock), support electronic control of the lens aperture
and the lenses usually contain autofocus motors.

So. When it
comes to adapting FD lenses to EOS cameras the key difference here is the
lens register incompatibility. Adapters
for Canon FD-mount lenses, including the two adapter models that Canon themselves
sold at one point, must deal with this incompatibility somehow. Either they
contain optics (glass lens elements) to compensate for the register difference
and retain infinity focus or else they dont contain optics and dont
retain infinity focus. Theres no way around this.

Canons
optic-containing FD-lens-on-EOS-body adapters were basically small teleconverters
(1.26x) and only worked properly with a number of their longer telephoto lenses
- effectively transforming them into longer slower lenses - you lost 2/3 stop.
The full official list of compatible lenses is:

The adapters
cant be used on other lenses because they have large protruding front
elements which physically interfere (collide) with the rear lens element on
other FD lenses. They were known as the Canon Lens Converter FD-EOS
and were not sold to the public - only accredited professional photographers
could acquire them through Canon Professional Services. Apparently they were
intended to help pro photographers who had a big investment in huge Canon
FD telephoto lenses make the transition to the EOS system. These adapters
are thus quite rare, specialized and hard to find today. When they appear
on auction sites they tend to have rather high winning bids.

Third parties
have made similar glass-containing adapters, but by contrast their optics
are usually poor and thus image quality tends to be generally low.

Canon and others
also made FD lens to EOS body adapters which do not contain optics, but as
noted above, you lose infinity focus and so such simple metal ring adapters
are really only useful for closeup applications. In fact, the Canon model
(which was sold to the general public) was marketed under the name Macro
Lens Mount Converter FD-EOS. Theres also a more subtle metering
isue with this sort of adapter - see the potential problem section below.
If youre interested in this approach you could always cobble
yourself together an adapter using old body caps and other parts.

But in short
I dont think that adapting a Canon FD lens to an EOS camera is worth
the trouble for most situations. If you have a really expensive FD lens already
Id probably be more inclined to use it with an FD-compatible body such
as a nice used T90 or A1, unless you really need some unusual function supplied
by EOS bodies. Or you might want to do macro photography with your digital EOS
camera using old FD lenses.

So ironically enough,
EOS cameras usually work better with non-Canon manual-focus lenses than with
Canon manual-focus lenses. A shame, because Canon did build some very nice FD lenses years
ago, many of which are available quite cheaply these days because the FD system
was effectively orphaned by the introduction of EOS.

* Yes, it is
true that Canon developed three AC series lenses which were basically
FD mount lenses with autofocus motors built in. However, these lenses worked
only with the Canon T80 camera and were essentially an evolutionary dead-end.
So its not entirely inaccurate to say that all actual FD mount lenses
are manual focus only.

Focussing.

Anyway, once
youve got the lens attached youll obviously need to focus it.
Unfortunately, Canon EOS cameras were never really designed with manual lens focussing
in mind and so come equipped with focussing screens which lack any form of
manual focus assistance. By contrast, many cameras from the 1970s and 80s
featured split-circle and microprism focussing aids that simplified focussing
a great deal.

If your camera
has interchangeable focussing screens and if you find yourself doing a lot
of manual focus work you might want to consider investing in a screen that
can provide you with useful manual-focus feedback. The EOS 600 series, EOS
3 and all EOS 1 series cameras support interchangeable screens and split-circle
viewfinder screens are available. EOS 5/A2/A2E cameras supported interchangeable
screens, but no split-circle or other manual focus screens were ever made. The same with the EOS 5D - it has interchangeable screens, but no manual focus screens are forthcoming from Canon. However, users of recent digital cameras (EOS 10D, 20D, 300D and 350D) may find that a third party screen from Katz Eye or Haoda Fu will work. I have tried the Fu product and have a review of it on my Web site. I have not tried the Katz-Eye products.

Without a manual focus screen youre
just going to have squint through the viewfinder and hope. Or buy an actual
manual-focus camera that fits your lens. (or hack
your camera to have a split-circle viewfinder screen if youve got
a Rebel/EOS 1000 or Elan/100 camera) Note that the in-focus indicator in the
viewfinder doesnt work unless the camera can communicate with the lens
electronically, so you cant rely on that either, unless youve added a chip. (cloned versions of such chips are now available - a number of small retailers sell them on eBay, for those of a tinkering bent)

Finally, once
youve got your picture composed and focussed youll need to set
the exposure settings correctly, and for this youll need stop-down metering.

What
is stop-down metering?

Normally your
EOS camera will meter the light using the widest aperture possible for the
lens. (ie: the aperture diaphragm will be fully open - f/1.8 or f/3.5 or whatever
the lens supports) This lets you look through the viewfinder with the maximum
amount of light coming in. When you take a photo the camera quickly stops the lens down to your selected aperture just before opening the shutter. Canon EF lenses rely on electronically-controlled
aperture diaphragms, not mechanical ones operated by the camera body via a
cam or lever. As soon as the shutter has closed again and the picture has
been taken, the lens diaphragm pops back to the widest setting once more.

There are two
exceptions to this mode of operation. First, if your camera has depth of field
preview and you have it enabled, the camera will stop down the lens aperture
to whatever youve set it to be. And second, if you have a lens installed
that doesnt have an electronically-controlled aperture diaphragm the
camera will operate in stop-down metering mode - itll meter whatever
the lens is currently stopped down to. Hence stop-down metering.

All Canon EF
lenses contain a computer chip and various electrical connectors that allow
the lens computer to communicate with the cameras computer. Most third-party
lenses for EOS also have EF-mount compatible electronics. However, if you
buy a lens designed for another camera system and attach it to your EOS camera
using a lens mount adapter youre obviously not going to get the full
electronic control youd get with a true EF lens. In particular, if there
are no compatible electronics then neither autofocus nor electronically-controlled
apertures will work.

But
my EOS camera has no aperture lever!

Thats
correct. Canon EOS camera bodies, unlike older mechanical camera bodies, do
not have a mechanical lever to stop down the lens aperture when the photo
is taken. Thats why you need a lens capable of maintaining your selected
aperture setting at all times.

Some lenses
have manual and auto settings that you may need to adjust. Others have small
pins on the lens mounting ring that need to be held in at all times - check
to see if your adapter ring keeps the pin in place.

If you have
the manual for your lens see if theres a section on stop-down metering.
What you dont want is the lens operating under the assumption that a
mechanical lever inside the camera body will stop down the lens aperture at
time of exposure.

Old
and new stop-down metering styles.

The earliest
Canon EOS cameras and the later models handle stop-down metering in two different
ways. Its easy to tell which style your camera uses.

Turn your camera
on and remove any lens that may be attached to it. With no lens attached to
the bayonet press the shutter release down halfway and look at the aperture
setting in the viewfinder or top-deck panel.

If the display
reads 1.0 (or any number other than 00) then you have
the old stop-down metering style. Only a few ancient EOS film cameras use this method.

If the display
reads 00 then you have the new stop-down
metering style. Most EOS film cameras and all EOS digital cameras support this.

How
to use the really old-style stop-down metering style (display shows something other
than 00).

If you attach
a non-EF lens to your older Canon EOS film camera, the camera notices that the
lens doesnt have a working computer and goes into stop-down metering
mode. It nonetheless displays a full range of apertures, which you can set
from 1.0 to 32.

Set the cameras
aperture setting to 1.0 and leave it there.

Do not
set the cameras aperture value to match that of the lens. In fact, I
dont know why the camera lets you change the aperture setting at all,
since it only screws things up by overexposing the image.

If your lens
has an adjustable aperture (usually an aperture ring on the barrel) you must
do the adjustment on the lens itself, not the camera. This will obviously
vary the amount of light entering the camera. The camera reads it and meters
from that accordingly.

Now, since the
camera isnt capable of adjusting the aperture setting on the lens it
cant work in P (program), Tv (shutter speed priority) or PIC (icon)
modes with such a lens, but itll work just fine in Av (aperture priority)
and M (manual) modes.

In Av mode you
set the lens aperture using the lens aperture ring and the cameras aperture
setting to 1.0 and then camera will set a shutter speed automatically. In
M mode you set the aperture using the lens aperture ring and then set the
shutter speed on the camera yourself.

One other note
- some older EOS cameras have the annoying inability to remember aperture
settings when you switch from one mode to another. Some models, such as the
EOS 620/650 or 10/10s, automatically switch the aperture value to 5.6 whenever
you go into Av and M modes. Since the aperture value must be set to 1.0 when
using stop-down metering and a manual lens you have to dial the aperture back
every single time you enter either mode. This is particularly annoying
in M mode, because since these cameras lack a rear command dial you have to
hold the partial metering button whilst rotating the main dial. Oh, well.

How
to use the new-style stop-down metering style (display shows 00).

If you attach
a non-EF lens to your newer Canon EOS camera, the camera notices that the
lens doesnt have a working computer and goes into stop-down metering
mode, displaying the aperture value 00. This means that the camera knows that
the aperture setting is not under its control and will not let you set the
aperture electronically. This metering style makes a lot more sense than the
old method.

If your lens
has an adjustable aperture (usually an aperture ring on the lens barrel) adjust
it now. This will obviously vary the amount of light entering the camera.
The camera reads it and meters from that accordingly.

Now, since the
camera isnt capable of adjusting the aperture setting on the lens it
cant work in the Tv mode with such a lens, but itll work just
fine in the other creative modes - P (program), Av (aperture priority)
and M (manual). It doesnt really make sense to use the lens in any of
the PIC (icon) modes.

Set the aperture
using the lens aperture ring and the camera will set the correct shutter speed
in all other modes except M, where youll have to set the shutter speed
yourself. Generally, Av mode is probably the most convenient.

Potential
problems.

Stop-down metering
and manual focussing is pretty straightforward, but here are a handful of
potential pitfalls.

Clearance
problems.

Some users of
wide-angle manual lenses, such as the MC
Zenitar 16mm fisheye lens and various super-wide Leica lenses, have reported mirror clearance problems on some film and full-frame digital cameras. In
other words the rear element (rear-mounted filter in the case of the Zenitar
lens) protrudes out too far and physically blocks the cameras reflex
mirror. If the camera locks up with a blinking battery symbol when you try
to take a photo (and the battery is OK) then its likely that the lens
is blocking the mirror.

If this happens
you may have a fundamental incompatibility between the lens and camera body (particularly with Leica wide angles) or you may be lucky and may just have an issue with
the lens adapter. For example, the people with problems with their Zenitar
lenses seem all to be using an M42 to EOS mount adapter. My own Zenitar lens
has an actual EOS mount attached to it, not the M42 adapter, and Ive
had no mirror clearance problems. Its possible that the users of the
M42 to EOS lens adapters havent fully tightened the lens onto the adapter
ring, which could lead to problems. The EOS 5D camera is particularly vulnerable to wide-angle lenses colliding with its mirror, though I can report that the Zenitar fisheye works fine so long as you have the real EF mount on it - I cant say if the M42 to EOS mount adapter works.

Camera locks up with the manual focus lens installed.

Canon EOS cameras contain tiny switches in the lens mount which are used to sense the presence of a lens with autofocus electronics. If a manual focus lens trips this switch - perhaps through a badly designed lens mount adapter - then the camera will lock up because the switch is telling it to expect an electronic lens, but there are no electronics to be seen.

In such a case you may need to unlock the lens and rotate it very slightly so that the switch doesnt engage. If you do this dont turn it too far - or dont let it get bumped - because the lens might fall off!

A similar problem occurs if you install a teleconverter (extender) or extension tube between your manual focus lens and your camera. The camera will get confused and lock up because the autofocus lens switch is tripped but there arent any lens computers around. The workaround is the same as the above - unlock and rotate the lens and TC.

The 5/A2/A2E
(same film camera; different names for different markets) and earlier Elan 7/EOS
30/33/7 camera (same film camera; different names) do not meter correctly when
a manual lens is attached - theyre typically a few stops out. Its
really unfortunate that these cameras have this
flaw, but there you go. Fortunately it appears that later model Elan 7/EOS
30/33/EOS 7 cameras have the problem fixed, and I am told you can take your
camera to Canon to have to reprogrammed if you have one of the earlier models.
I dont have any details on this, however - you would need to contact
Canon.

If you know
German you could also have a look at this
page here, which provides some details on fixing the problems for the
adventuresome. Essentially the process involves cannibalizing an old lens
(the author used a defunct 35-70 zoom) and attaching the circuitry to an M42
adapter. If this seems too daunting you could simply try compensating
for the error manually, though I hear conflicting reports as to whether or
not the metering error is linear.

Dark image
in the viewfinder.

Well, thats
why I suggest you focus first! If youre setting the lens to a small
aperture you may find the image in the viewfinder is too dark to focus properly.

Metering
is slightly out.

Its possible
you may have problems achieving completely accurate metering with your camera
and manual lens. Since the camera has no way of obtaining lens characteristics
from the lens it doesnt know what sort of metering compensation to apply.
See also the issue with the FD adapter mentioned below or the issue of the
Elan 7/EOS 30/33/7 mentioned above.

Its probably
worth shooting a few test frames on digital, or a test roll of film, when you first put a manual lens on your
camera to see if the camera is metering correctly for the lens. My Zenitar
fisheye lens, for example, is generally OK but seems to be about a half
stop out if wide open or fully stopped down. I find that Lensbabies manual lenses tend to be a whole stop overexposed.

Finally, it
seems youre probably best off using partial metering (or centre-weighted averaging) over evaluative.
Partial metering meters a smallish area at the centre of the frame rather
than trying to apply metering algorithms based on metering individual segments
of the whole frame. I dont know if its because the evaluative
algorithms rely on information on the lens or what, but partial seems to be
more accurate from the limited tests Ive done. Either way, be sure to
test your own gear yourself first. See the note below regarding spot metering,
however.

Metering
problems with the Canon FD->EOS adapter.

Theres
a remark from Canons Chuck Westfall about a problem with the Canon FD-EOS
adapter and EOS cameras which lack interchangeable focus screens in the old
EOS FAQ. Look for section 9.18 - Why are there limitations on the
use of the FD->EOS Macro adaptor (sic) with some EOS Cameras?

Spot metering
problems.

Apparently,
and I dont have a camera for which this is an issue, some EOS cameras
with spot metering (eg: EOS 1 and 1n) cant handle spot metering correctly
with a manual lens adapted this way. Again, its probably worth doing
a little test in the various modes before taking that once in a lifetime shot.

No adjustable
aperture ring.

Some lenses
do not have an adjustable aperture. For example, mirror (catadioptric) lenses
have a fixed aperture - usually f/8. So the only ways to adjust the amount
of light hitting the film are to adjust the shutter speed, use different film
speeds or put a neutral density filter over the lens. This limitation results
from inherent properties of the lens, and not from any lens to camera body
adapter.

Incompatible
aperture ring.

As noted above,
some lenses, such as older Pentax M42 threadmount lenses, cant be stopped
down manually or else require an EOS->M42 adapter that holds down a pin
on the lens mount. Later Pentax threadmount lenses have a manual/auto switch
that lets you determine whether or not the lens aperture changes when you
turn the aperture ring.

If you cant
stop down the aperture you may be forced to shoot wide-open all the time,
which would be unfortunate since thats where the lowest quality of the
lens tends to be.

Telescope
focussing problems and the Scheiner disc.

It can be really
hard to focus a telescope properly, especially when youre peering through
a camera viewfinder. One useful trick is to cover the end of the telescope
with a piece of cardboard or similar opaque that has two round holes cut into
it. When you look through the viewfinder at a bright object youll see
two dim ghost images. Adjust the focus until the images overlap, then remove
the cardboard. Youll probably need to experiment with hole size and
placement for this to work well. A typical suggestion is to put in a pair
of holes each about 1/3 to 1/4 the size of the lens diameter, separated if
possible by a distance equivalent to that hole diameter. (ie: both holes should
be the same distance from the centre of the lens)

Such discs
are called Scheiner discs, as they were invented by Galileos sometime
adversary Christoph
Scheiner (1573-1650), apparently in 1619. Theyre also commonly referred
to as Hartmann or Shack-Hartmann masks, though Hartmann masks usually contain
more than two holes.

M42/T-mount
confusion.

The M42 threaded
mount (the universal screw mount popularized by the Pentax Spotmatic
camera) is similar to the Tamron T-mount - both mounts are 42mm in diameter.
However, M42 lenses use 1mm threads and T-mounts use 0.75mm threads. This
stupid decision on the part of Tamron (the M42 mount predates the T-mount)
means its quite easy to wreck a lens or camera if you try to force the
wrong type of lens onto the wrong type of camera, even though they look like
they should fit.

So be very careful,
and double-check with your dealer before you buy the lens adapter. Spotmatic-style
lens mounts are usually called M42 or M42 body mounts, but sometimes T-mounts
are referred to as M42 as well.

Battery drain
with long exposures.

Most EOS cameras
require battery power to keep the shutter open. This is obviously a problem
for astrophotography, a field in which very long (many hours) exposures are
quite common. Itd be really frustrating to have a picture turn out poorly
because a battery drained flat in the middle of a 5-hour exposure. The risk
is compounded by the fact that battery performance drops dramatically in cold
temperatures, and its often very cold at night. Bob Atkins Web
site has a useful
list of the approximate battery life times for a number of EOS cameras,
but note that it hasnt been updated to include newer EOS cameras. For
example, the EOS 3 supposedly requires minimal battery power to stay open,
but I dont have any hard data on it.

If battery drain
is a problem for you, consider buying an EOS
10 or an older EOS 600-series camera. These first-generation EOS models
dont need power to keep the shutter open. In fact you can disconnect
the battery once the exposure has started and theyll stay open.

Im not
sure about lenses. All EF lenses contain an electrically-controlled aperture
diaphragm that requires power to open and close. Ive tested all my lenses
(use depth of field preview to stop down the lens, then remove the battery)
and they dont seem to require any power to keep stopped down, but its
entirely possible that this varies on a lens by lens basis - test first. Obviously,
a manual focus purely mechanical lens will not require any battery power at
all to keep stopped down.

Adapting
EF lenses to non-EOS cameras.

Well, having
spent all that time covering the use of non-EF lenses with EOS cameras, how
about the reverse? Can you take your favourite Canon EOS autofocussing lens
and put it onto your Canon FD or Nikon or Pentax or whatever camera?

The short answer
is no.

The long answer
is that its probably sort of technically possible, depending on the
camera body, but nobodys done it, as far as I know. More trouble than
its worth.

As noted earlier
in this document, EF lenses have complex electronic systems which are used
to transmit aperture and other data from the camera to the lens. They dont
have simple mechanical rings or levers to adjust the aperture. So using an
EF lens without working electronics would be difficult. You could shoot wide
open. Or you could stick the EF lens onto an EOS camera, set the aperture
to whatever you want it to be and then remove the battery from the camera.
All the Canon EF lenses Ive used will maintain their currently set lens
apertures when you disconnect the power.

In theory I
suppose that somebody could reverse-engineer Canons lens mounts and
design an adapter gadget to send the proper electronic signals to EF mount
lenses for manual aperture control, but it wouldnt be easy to do and
it almost certainly would not be cost-effective to manufacture as a commercial
product.

Focussing, on
the other hand, shouldn't be a problem. Aside from a handful of A series (autofocus-only
EF) lenses sold briefly in the late 1980s, nearly all Canon EF lenses can
adjust focus manually by rotating a focus ring. Most are mechanically linked
- either the focus ring turns a mechanical geartrain directly when the lens
is in manual-focus mode or, in the case of USM (ultrasonic motor) lenses with
FTM (full time manual) they turn a simple clutch ring setup. One exception
is a handful of early USM lenses, primarily longer telephotos, which used
a manual focus system that requires power. Such lenses, such as the 28-80
2.8-4L USM and the 85mm 1.2L USM, have a manual focus ring which instructs the USM drive to rotate.
These lenses couldnt be used, of course.

Finally, theres
the matter of the actual adapter ring. And this is where things are very problematic.
As far as I know no company has mass-produced adapter rings that let you attach
EF lenses to other camera systems. The EF lens mount is quite large; larger
than anyone elses 35mm lens mount. So youd need an adapter which
adapts a large lens to a smaller camera mount. Then youd probably need
optics to compensate for the register differences. If you hear of such a thing,
please let me know!

More
information.

Robert Monaghan
maintains a huge page on the subject of lens adapters:

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